This invention relates generally to CRT protection circuits and particularly to a circuit arrangement for preventing "burning" of the phosphor in the center of the cathode ray tube (CRT) faceplate.
The problem of phosphor burn due to an undeflected CRT electron beam impinging upon the phosphor screen is well-known in the CRT art. The difficulty arises when the beam deflection circuits are disabled while the video drive and high voltage circuits are still functional. In that event, the electron beam is no longer deflected over the phosphor target area but remains stationary and impinges upon a small freefall area in the central portion of the tube. The result is that the phosphor in that portion of the screen may be permanently damaged by the excessive energy dissipation. Two general approaches have been taken in the prior art to prevent the occurrence of phosphor burns upon turn-off of the apparatus that contains the CRT. One involves driving the CRT into heavy conduction while the deflection circuits are still active to assist in dissipating the high voltage energy stored by the large capacitance of the CRT. The other involves cutting off the CRT electron beam, and keeping it cut off, until the energy in the high voltage section has been dissipated to a safe level.
U.S. Pat. No. 4,275,338 discusses a number of prior art approaches including: maintaining residual operating voltage to permit continued functioning of the deflection circuits after turn-off of the CRT display; turning on the CRT electron beam more heavily to dissipate the stored high voltage energy prior to collapse of the beam deflection voltages; utilizing special logic circuits to completely blank the CRT electron beam upon sensing the absence of deflection voltage; and turning on a discharge transistor that is connected across the video output stage to drive the CRT into very heavy conduction upon turn-off of the display. That patent discloses a diode network coupled to the B+ supply of the video amplifier for charging a capacitor. When the power supply is turned off, the charged capacitor maintains the video amplifier operating voltage which keeps the cathode of the CRT positive with respect to the grid voltage. Thus, the CRT is held in cutoff for a time period after turn-off of the power supply.
U.S. Pat. No. 4,390,817 uses a bypass capacitor that is coupled to the control grid of the CRT. The circuit arrangement is such that negative cutoff potential is applied to the CRT control grid when the receiver is turned off.
U.S. Pat. No. 4,217,525 includes a transistor that is driven into cutoff when the receiver power supply is turned off. In so doing, the potential on the control grid of the CRT is elevated to drive the CRT into heavy conduction to discharge stored energy prior to collapse of the deflection system.
It will be appreciated that the high line video amplifiers in a monitor may be utilized with different low level video inputs, i.e., different signal inputs. For example, it is quite common to have a red, green and blue (RGB) video input as well as a composite R, G and B video input. The particular circuit arrangements in the sources determine whether the high level video output stage will be turned on hard or turned off when the main power supply is turned off. Also, with switch mode power supplies being extensively used, the sequence of voltage shutdowns when the power is turned off may result in situations where phosphor burn conditions may exist. A supplier of video monitors thus has a great interest in a circuit arrangement that effectively precludes the danger of phosphor burn upon turn-off of the monitor, irrespective of the type of video input that is connected to the monitor.
The circuit of the invention assures cutoff of the CRT, regardless of the type of low level video circuit that is coupled to the high level video circuit by both maintaining an operating potential on the high level video amplifier to keep the cathode of the CRT positively biased with respect to the control grid for a predetermined time after turn-off and by applying a negative voltage to the control grid of the CRT. This combination of a positive voltage applied to the cathode and a negative potential applied to the control grid assures that the CRT is driven into cutoff and held there for a sufficient time to preclude the danger of phosphor burn upon turn-off of the power supply.